You've had major abdominal surgery during which all sorts of abdominal muscles were cut. These take months to fully mend. The pain is your body's way of telling you that you're overdoing it - listen to it!
I still get occasional muscular twinges if I overdo things 21 months after having robotic surgery to remove my left kidney. After such major surgery I don't think that things ever go entirely back to the way they were before, unfortunately.
Best wishes,
Chris